1. Field of the Invention:
The present invention relates to apparatus for manufacturing tubular thermoplastic film and particularly to a variable size deflector which facilitates the starting process and provides more efficient control of internal cooling gas.
2. Statement of Prior Art:
In the known process for manufacturing tubular thermoplastic film, a plastic tube is extruded through a vertical die and inflated to a desired diameter by the pressure and volume of gas or air flowing in a central conduit. The enlarged tube is cooled as it passes over a disc which directs internal air at the film while additional external air is directed at the outer surfaces. The tube is then drawn upwardly through and flattened by a pair of rollers. An example of this prior art process is found in U.S. Pat. No. 2,966,700 issued Jan. 3, 1961. It has also been known to utilize a plurality of axially spaced internal discs to provide improved stability and control of cooling air, as shown in U.S. Pat. No. 3,976,411 issued Aug 24, 1976.
A serious problem, however, is encountered in the starting process when the thin heated plastic tube from the narrow extruder die is pulled over the wider diameter disc to the windup rollers. The ratio of the maximum diameter of the tube to the diameter at the exit of the extruder die, known as the "blow up ratio" is generally in the order of from 1.5 to 4:1, with the deflector disc being only slightly smaller in diameter than the maximum dimension of the tube. It is therefore extremely difficult to pull the small size extruded tube over the large deflector disc when starting, particularly since the tube cannot trap air and be blown up to a larger size until passing through the nip rolls which collapse and draw the tube upwardly.
Additionally, when using a fixed diameter deflector disc, the size of the tube cannot be varied without replacing the deflector disc. In order to do so, the entire operation must be shut down, and then the entire starting process repeated with all the difficulties previously described.
Known possible solutions for this problem include an internal pneumatic inflatable forming device which can be expanded from a narrow to wider dimensions to control the size of the tube, as shown in U.S. Pat. No. 3,307,219 issued Mar. 7, 1967, and a mechanically expandable device having a plurality of arcuate nozzles disposed on the ends of movable linkages or plungers, as in U.S. Pat. No. 3,810,725 issued May 14, 1974. The first type, however, is capable of only limited expansion and cannot provide adequate control of gas flow as is obtainable with a thin disc type deflector, while the second device does not provide a continuous smooth peripheral surface in the expanded position. In addition, the arcuate nozzle sections provide a circular contour only at the contracted diameter position and cannot match the desired curvature of the plastic tube at the maximum diameter.